Large capacity storage tanks are widely used in many industries particularly in the processing plants such as oil refinery and petrochemical industry. They are used to store a multitude of different products such as raw material and finished products. Prediction of the residual stresses is of the most important problems in the tanks manufacturing. One of the most pronounced effects of residual stress is to accelerate, or in some cases retard, crack growth in structures subject to cyclic loading. This dissertation presents numerical study of multi-pass welding and thermal stress relief treatment of an oil storage tank plate specimen, the welding phase contains two parts; heating up part and cooling part. In the first part the material is heated to its liquids temperature. In the cooling part it is cooled down to the ambient temperature. This leads to develop high residual stresses in weld zone i.e. fusion zone and heat affected zone. The thermal stress relief treatment phase for a finite time interval the weld zone is heated up to such a high temperature that activates creep mechanism in the material. This is one of the most effective ways to relief welding residual stresses. Finite element method is employed to model and analyze both the welding and thermal stress relief processes for multi-pass butt-welded AISI 304 low carbon steel of an oil storage tank plate specimen. The main goal of this dissertation is to investigate the effect of welding parameters such as (heat input, welding speed), wall thickness and preheating. In the second part the effect of thermal stress relief treatment parameters, including holding time and temperature, on the distribution of welding residual stresses are investigated. In this simulation sequential coupled thermal-mechanical model is developed in welding simulation and thermo physical and mechanical properties of material used for welding and thermal stress relief treatment simulation are depending on the temperature. In the thermal analysis of welding phase, the weld heat source model based on the Goldak’s double ellipsoid power density distribution was applied. After the thermal analysis, the mechanical analysis is performed. Then the stress distribution of the specimen has been transferred to a second analysis for stress relaxation modeling. Norton law is used to investigate creep in stress relief process. Whole the processes are simulated on ANSYS commercial software developing related ANSYS codes. Keywords : Welding simulation, Residual stresses, Heat source model, Thermal stress relief, Stress relaxation, Creep, Holding time and temperature.
